1. Technical Field
The invention relates to transistors of a semiconductor device and methods of forming the same, and more particularly, to transistors of a semiconductor device having a punchthrough protection layer and methods of forming the same.
2. Discussion of the Related Art
Generally, a transistor includes a word line pattern, source/drain regions and a channel region. The channel region is disposed in the semiconductor substrate under the word line pattern, and the source/drain regions are disposed in the semiconductor substrate so that they overlap the word line pattern. The word line pattern is also disposed on the semiconductor substrate along with the source/drain regions and the channel region. The channel region as well as the word line pattern are reduced in size with reduction of a design rule of a semiconductor device. This reduction tends to create problems, such as a punchthrough between the source and the drain region, as explained below.
To cope with these problems, there has been much research on the semiconductor device to improve characteristics in spite of the reduction of the design rule. One research effort proposes a transistor including a channel-portion hole disposed in the semiconductor substrate, and a word line pattern filling the channel-portion hole. The channel-portion hole has a trench shape. The word line pattern provides a channel region along the semiconductor substrate defining the channel-portion hole. The channel region around the channel-portion hole is longer in length than that of the channel region under the word line pattern on the semiconductor substrate.
However, although the transistor having the channel-portion hole can increase an integration degree of a semiconductor device with the increase of the length of the channel region, a punchthrough between the source region and the drain region associated with the reduction of the design rule is not improved. This is because the reduction of the design rule of the semiconductor device decreases a diameter of the channel-portion hole and concurrently decreases the distance between the source region and the drain region along the channel region around the channel-portion hole. Therefore, the transistor having the channel-portion hole requires a method of improving the punchthrough of the source and the drain regions.
On the other hand, U.S. Pat. No. 6,423,618 to Ming-Jang Lin, et al. (the '618 patent) discloses methods of manufacturing a trench gate structure.
According to the '618 patent, the method includes sequentially forming an epitaxial layer, and a first and a second dielectric layer on a semiconductor substrate. The epitaxial layer has a base region and a source region overlapping the base region. A trench is formed in the epitaxial layer to sequentially penetrate the source and the base regions along with the second and the first dielectric layers.
The method includes forming a third dielectric layer at a lower portion of the trench, and forming a gate oxide layer conformably covering the trench. A polysilicon layer is formed on the second dielectric layer to conformably cover the trench, and a fourth dielectric layer is formed on the polysilicon layer to fill the trench. Subsequently, the fourth dielectric layer and the polysilicon layer are partially removed so that the base region has the same level as the fourth dielectric layer and the polysilicon layer.
However, the method includes performing an etching process on the epitaxial layer to form a transistor having a trench gate structure. The transistor may have an unstable interfacial state along the epitaxial layer defining the trench due to the etching process. The unstable interface of the epitaxial layer becomes a part of a channel region of the transistor, thereby causing a leakage current during the operation of the semiconductor device. Further, the method discloses a method of forming a transistor using different oxide layers on a bottom and a sidewall of the trench as the gate oxide layer, but the oxide layers inside the trench may increase the leakage current and decrease a breakdown voltage of the gate oxide layer due to the different oxide layers.